Of the objectives identified for the entire duration of the project, those to be pursued in the fourth year are: 1. Completion of the characterization of the canine pepsinogens. They will be identified by their electrophoretic behavior on polyacrylamide slab gels and isolated principally by chromatography on DEAE-cellulose. 2. Isolation of the canine pepsins following activation of the zymogens, and characterization of the pepsins' catalytic specificities for small peptide substrates that have been useful in distinguishing among acid proteases isolated from pig and cow stomachs (pepsin, chymosin, gastricsin). 3. Beginning of amino acid sequence studies on the canine pepsinogens using semiautomated solid phase methodology. 4. Continuation of studies on activation of porcine pepsinogen using a spin-labeled derivative whose ESR spectrum changes during activation, and by observing the stopped flow kinetics of the UV spectral and fluorescence changes during zymogen activation.